1. Field of the Invention
The present invention relates generally to testing and evaluation of electrical or magnetic devices, and more particularly to methods of increasing yield and quality of devices by considering multiple performance parameters.
2. Description of the Prior Art
Electronic components such as integrated circuits and disk drive heads often fail due to flaws or marginal performance resulting from variability in the manufacturing process. Components are shipped or incorporated into larger assemblies. Certain qualities of these components influence the performance of the completed assembly.
In the semiconductor field, to minimize failures in the field, manufacturers routinely test components to screen out defects and ensure quality levels shipped to the consumer. A certain quantity of parts shipped or incorporated into larger assemblies will eventually fail to function after running in a use condition for some period of time, due to “reliability defects” which are not apparent until after the parts have been used for some period of time. It has become common practice to use an acceleration technique called “burn in” as part of their production test procedures. Burn-in generally consists of exposing the part to extremes of voltage and temperature, in order to screen out many reliability defects in their products before shipping the parts to the consumer, with the expectation that a lower quantity of reliability failures will occur.
In order to reduce the time and expense of component burn-in, “binning” systems have been developed, by which components are separated into separate bins based on certain criteria that reflect on the expected reliability of the components. For example, it has been discovered that the likelihood of a component that passes a particular type of testing having a latent defect increases with the number of neighboring components that fail this same type of testing. By “binning” those components that pass this particular testing into separate groups depending on how many of its neighbors failed this same testing, the components are separated into groups expected to have greater or lesser degrees of early life reliability.
In the disk drive industry, heads may be built into several models of hard disk drives, each with different relative sensitivities of soft error rate (SER) to various parameters measured to characterize the heads. Heads can be segregated and used in the model in which their SER is likely to be best. This segregation is referred to as binning. Heads which are very likely to produce poor SER in the disk drive may be scrapped at component level, saving the expense of further processing and assembly.
As there are multiple factors that can be observed as the criteria for the binning process, the process can become very complex, with multiple parameters to be considered. As it is important that potentially defective components be screened out, it is also important that marginal components be evaluated in a manner which does not exclude too many which may in practice turn out to be acceptably reliable. As an example, a disk drive read head may have, among other qualities, the qualities of 1) amplitude, meaning the amplitude of the signal it is capable of producing in a uniform quasi-static magnetic field, and 2) recession, meaning the relative amount of distance that the head is positioned relative to the data medium that it is reading. In prior art binning systems, components may be screened for each of these parameters, perhaps given a rating from 1-10 on the scale of the qualities. There may be a cut-off point established for acceptability, so that, for instance, for amplitude, a relative value of 5 is required, so that a component with a rating of 6 is acceptable, and one with a rating of 4 is rejected. Separate criteria for recession may be also established so that any component with a recession value of 5 or better is acceptable, meaning that it is not receded from the disk surface too much. Components which are receded may have values of 4 or less as their recession distance increases. A prior art binning system may look at each of these factors and assign a “go, no-go” evaluation based on each of these independent parameters.
However, there will exist some components in which there is a compensation effect in these two factors that can be considered. For example, a component may have an unacceptable “recession value” of 4, but have an “amplitude value” of 7, meaning that the head recession places it farther away than is desirable, but the amplitude of signal generated may be large enough compensate for this, so that, although the magnetic field that drives the head may be attenuated by too great a distance between the sensor and the media, the signal that it produces is large enough that the end result will be acceptable. A system that rejects this component on the basis of the single “recession value” will be wasting a component that in practice may function perfectly.
In the current manufacturing process, there may be 28 or more such parameters to take into account. It can easily be seen that if each component is assigned a pass-fail judgment based on these parameters considered independently, many components will fail needlessly and yields will be lower than necessary.
Thus, it can be seen that needs exist for improved systems and methods for determining the reliability of electronic components which considers the interaction of multiple parameters and their interactive effect on overall reliability.